Self configuring and optimisation of cell neighbours in wireless telecommunications networks

ABSTRACT

The present invention relates to a cellular communications network and in particular to the problem of identifying cells for neighbour lists, or handover candidates when the cells are identified by a limited number of non-unique identities. The present invention includes a method for a radio base station, wherein the base station receives reports from one ore more terminals in neighbour cells quality and non-unique identities. The match is sought in a neighbour cell list non-unique identity with a unique cell identity. If the match cannot be made without ambiguity, the base station orders the terminal to detect the unique cell identity. If the reason for the ambiguity is the particular cell is not included in the neighbour cell it is included when its unique identity has been established. The uniquely identified cell is used if a handover is decided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/084,884, filed Apr. 12, 2011, which is a continuation of U.S.application Ser. No. 11/773,752, filed on Jul. 5, 2007, which claims thebenefit of International Application No. PCT/EP2007/01737, filed Feb.28, 2007, the disclosures of which are incorporated herein by reference.

FIELD OF INVENTION

The present invention relates to self-configuring and optimisation ofcell neighbours in wireless telecommunications networks.

BACKGROUND OF THE INVENTION

FIG. 1 of the accompanying drawings illustrates a wirelesstelecommunications network, which support communication with terminalsin a number of cells (A, B, C, D) each of which is served by a radiobase station 2. Each communication cell covers a geographical area, andby combining a number of cells a wide area can be covered. A mobileterminal 4 is illustrated communicating in cell A, and is able to movearound the system 1.

A base station 2 contains a number of receivers and transmitters to giveradio coverage for one or more cells. Each base station 2 is connectedto a network “backbone”, or core network infrastructure (not shown),which enables communications between base stations and other networks.The example system of FIG. 1 shows one base station per cell.

An important concept in such a network is the cell and its neighbours.During a call a mobile terminal 4 typically is moved in the geography,and when so doing leaves a first cell and enters a new cell thatneighbours the first cell. The cell may be changed several times and theprocess of changing the cell that supports a radio link with theterminal is called handover. A list of the known neighbours, the socalled “neighbour cell set”, is important both for the network 1 and forthe mobile terminal 4 to enable reliable handover between cells. Thenetwork 1 can store information relating to a set of neighbour cells foreach cell in the system. Evaluation of the best cell for supporting aradio link with the mobile terminal is based upon measurements made bythe terminal on the serving cell and on other cells than the servingcell. The neighbour cell list is needed for mapping measurements andhandover decisions to a target cell identity and possibly applyingspecific parameters or rules for the target cell. It will be readilyappreciated that the cell boundaries are not sharply defined, but willin practice be somewhat blurred as the range of the base stations willoverlap with one another.

In existing systems, the mobile terminal, 4, detects and measures celloperating parameters for neighbouring cells by measuring on theirbroadcast channels. One measured operating parameter is a cellnon-unique identifier which typically consists of a physical layeridentifier such as a scramble code which is non uniquely assigned to thecell. Operating parameters also relates to the signal quality of theneighbour cell such as signal strength, signal quality and timinginformation. When the quality of a neighbour cell is considered betterthan the current serving cell, a handover from the serving cell to thechosen neighbour cell is executed by the network. The neighbour cellthen becomes the serving cell for the mobile terminal.

Typically in a WCDMA (wideband code division multiple access) system,the mobile terminal detects Common Pilot Channel (CPICH) transmissionsfrom surrounding cells, in order to determine id (scramble code) andtiming information.

When the mobile reports the neighbour cell signal quality measurementsto the network, the cells' respective identities become important.Typically, cell identities are reused for more than one cell. The reuseof identities means that cells may be confused with one other, since theserving cell may have neighbour cells sharing the same identityinformation.

In the cells are also broadcasted unique cell identities. The uniquecell identity is carried on the network layer. Its main use is forspecial purpose terminals, that are used by an operator of the systemfor testing and tracing of problems in the network. The unique cellidentity is not repeated as often as the non-unique cell identity and ismore complicated for a terminal to detect. This is in contrast to thephysical layer, that is physical data needed for supporting the radiolink. Since the cells' physical layer identifiers are non-unique,populating and maintaining the neighbour cell sets can never be fullyautomatic. Human efforts are needed to resolve conflicts where theserving cell has multiple neighbours using the same non-uniqueidentifier. A further problem is handover failures owing to thecandidate cell has been incorrectly identified.

SUMMARY OF THE PRESENT INVENTION

The aim of the present invention is to secure handovers to be performedto the right cell. It solves the problem with a method for a radio basestation or for a node controlling a radio base station serving a firstcell and comprises the steps of, receiving from a terminal a non-uniquecell identity of a second cell and a measure on the quality of thesecond cell, determining if the second cell can be unambiguouslyidentified by the information in a neighbour cell list, and if ambiguityin the second cell identity is determined, ordering the terminal toidentifying and reporting on the unique second cell identity, andassociating the non-unique identity with the unique identity.

In a first embodiment ambiguity on the second cell identity isidentified if the non-unique cell identity is not included in theneighbour cell list. The association is then made by including thesecond cell non-unique cell identity and the unique cell identity in theneighbour cell list. In a second embodiment ambiguity on the second cellidentity arises if two unique cell identities in the neighbour cell listhave the same non-unique identity. The association with the unique cellidentity with the non-unique cell identity is then made for the specificradio link with the terminal for a period. The period is typicallystarted with the signal strength of the second cell exceeded apredefined threshold value, and ending when a handover is performed orthe second cell signal strength decreases under a second thresholdvalue.

The invention also relates to a radio base stations or a nodecontrolling radio base stations and that is adapted for performing themethod.

The invention also includes a method for mobile stations that measuresthe signal quality on broadcast channels in other cells than the firstcell, detects the non unique cell identity on broadcast channels with aquality exceeding a threshold value, transmitting information on thesignal quality and second cell non-unique identity to the serving celland upon receiving a command, detecting the unique cell identity on abroadcast channel and transmitting the unique cell identity.

The invention further relates to a mobile station adapted for performingthe method.

An advantage of the present invention is that creation and or updatingof a neighbour cell list can be made automatically by the system basedon the measurements and cell identity information received from theterminals. Thereby, it is not necessary for humans to plan and maintainthe neighbour cell lists.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cellular wireless telecommunications network;

FIG. 2 illustrate steps of method for a base station or a base stationcontroller.

FIG. 3 illustrate steps of a method for a terminal.

FIG. 4 a is a block diagram of a mobile terminal.

FIG. 4 a is a block diagram of a base station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic problem of the present invention is to make a positiveidentification of a neighbouring cell that is ambiguously detected by amobile terminal. The ambiguity may arise when a cell, not previouslydetected, is reported by a mobile terminal, or when two or more cellsshare the same non-unique cell identity and the two cells. An ambiguitywill then arise on what cell that has been measured.

The problem exists in several systems, for example GSM, WCDMA andeUTRAN. In GSM and WCDMA a list of neighbour cells is created for eachcell in the system by a human or at least controlled by a human. In sodoing any collisions in the non-unique cell identities of the neighboursshould be detected and possibly solved by reallocating the non-uniquecell identities. It is cumbersome to establish and update the NCL(neighbour cell list) and the allocations of non-unique cell identities,especially when the capacity of a network is increased by the additionof further cells.

For WCDMA and eUTRAN it is desired that the NCL can, if not becompletely created automatically, at least be updated automatically bythe system based on measurements made by terminals on cells other thanthe cell serving the terminal. For that reason the present inventions isin particular important for the WCDMA and eUTRA, however, it may well beimplemented also in other systems such as the GSM.

The inventions will primarily be implemented in a network node thathandles the NCL and decides on handovers. In the eUTRAN this is the taskof the eNodeB which is a base station, in the GSM it is the BSC (BaseStation Controller) and in the WCDMA it is the RNC (Radio NetworkController) that handles the NCL. Both the BSC and the RNC have thefunctions of controlling the base stations and their operation on thevarious cells of the networks, and the term base station controller inthis application refers to a node having the functions of the BSC or theRNC.

FIG. 2 is a flow chart of the steps performed by a a radio base station,2, or a base station controller. For facilitating the description onlythe base station, 2, is mentioned when describing the method. It shouldbe understood that the steps are alternatively carried out by the basestation controller. In the first step 201 a communications link with aterminal, 4, in a first cell is supported. The base station receives;see step 202, a report from the terminal on a quality measure and anon-unique cell identity of a second cell. Next, 203, the base stationretrieves the NCL to match the non-unique cell identity with a uniquecell identity. If, see step 204, the match is made without any ambiguitythe method is ended with respect to the second cell. If howeverambiguity arises in the match, the base station instructs, 205, theterminal to detect the unique cell identity of the second cell. The basestation receives the unique cell identity, 206, and in a last step, 207,the non-unique cell identity is associated with the unique cell identitywithin the NCL.

There are several reasons why the ambiguity may arise in the matchbetween non-unique cell identity and the unique cell identity. Thefirst, and basic reason is the second cell is not included in the NCL.Another reason is the NCL includes an indicator of suspected ambiguity.The indicator may have been added to the NCL prior to the method isbeing performed. The reason may be handovers to the identified cell havefailed. The failure may be detected as terminals have re-establishedcontact with the first cell as serving cell during handover processes,or the target cell has not sent notification to the first cell asserving cell. A further reason for ambiguity is two or more unique cellidentities share the same non-unique cell identity.

If the reason for the ambiguity is that the second cell with its uniqueand non unique cell identities is not included in the NCL they are addedto it in the last associating step, 207.

The base station frequently receives, 202, reports from the terminal onmeasures made on other cells, and then not only the second cell. Whenthe base station has received, 206, the second cell unique identity asdetected by the terminal it shall not instruct the terminal to detectthe second cell unique identity when again receiving, 202, a qualitymeasure from the terminal, not even if the ambiguity in the NCL ismaintained. Therefore, the associating step, 207, shall associate theunique cell identity with the non-unique cell identity for thisparticular terminal for a specific period or as long as the signalquality of the second cell exceeds a predefined threshold. Theassociation with the particular terminal is not made in the NCL, insteada temporary NCL is used.

As long as the second cell unique cell identity is associated with theterminal, if handover to the second cell is decided it will be directedvia the associated second cell unique identity. Thereby the second cellwill be instructed to prepare a handover of the terminal, before theterminal itself is instructed to make a handover to the second cell.

In addition to the requirement of ambiguity in the match of unique cellidentity to the non-unique cell identity, the further requirement of thesecond cell quality measure exceeding a threshold value can be addedbefore the terminal is instructed to detect the second cell uniqueidentity. The reason for adding the further requirement is to avoidexpense of the terminal performing the detection. The unique cellidentity is transmitted from base stations at a much less frequentinterval than the physical layer identity. In order to receive anddecode this information, the mobile terminal 4 may have to shortlyinterrupt its communication with the serving cell.

Moreover, some filtering of the measurement data may be needed beforethe second cell is added to the NCL in the associating step 207. Forexample the second cell is not included until it has been reported bytwo or more mobile terminals. Thereby avoiding adding a distant cellthat was detected under exceptional propagation conditions, for examplefrom a mobile terminal 4 located in an aircraft.

Additionally in step 207, cell lookup maps the unique cell identity(UCID) to the address of the realising node of that cell. For example,in LTE, this can be an ordinary DNS, mapping the cell identity to an IPaddress. The IP address in turn points to the RBS realising the cell.

FIG. 3 is a flowchart of the steps to be performed by a mobile terminal.In a first step 301, the terminal is in communication with the firstcell over a first radio link. In the next step, 302, the terminalmeasures the quality of a broadcast channel transmitted in a secondcell, and detects, 303, the second cell non-unique identity as sent onlayer 1 communication. In a fourth step, 304, the terminal transmits thesecond cell non-unique identity, and quality measure over the firstlink. The quality measure and the non-unique identity are tied togetherin the transmission. In a following step, 305, the terminal receives aninstruction over the first link to detect the second cell uniqueidentity. Next, 306, the terminal detects the second cell uniqueidentity as broadcasted on network layer. In the last step, 307, theterminal transmits the second cell unique identity over the first link.

The terminal 4 scans the spectrum to find broadcast channels ofpotential neighbour cells. The ability to detect and measure a broadcastchannel depends on the sensitivity of the hardware in the terminaltransceiver and of the broadcast channel power relative to theinterference level. A further prerequisite for detecting the second cellidentity of the potential neighbour cell in step 303 may be added, andthat be the broadcast channel reception quality as detected in step 302,exceeds a threshold value. If the non-unique cell identity is notdetected, the second cell is of course not informed to the network, instep 304.

FIG. 4 a is a block diagram of parts of a mobile terminal 4 that areessential for the present invention. The mobile terminal 4 comprises acontroller 42, a man machine interface (MMI) 44, a radio transceiver 46,and an antenna 48. The controller 42 of the mobile terminal 4 serves tocontrol communications with the base station 2 via the transceiver 46and antenna 48, over the air interface 6. This means the controller 42have the function of receiving control messages from the base station,and control the operation of the mobile terminal in accordance with thecontrol messages. Data detected by the transceiver passes throughcontroller 42. The controller reads control information on the physicallayer. With respect to measures on other cells than the serving cell thecontroller 42 receives the measures on the reception quality andidentifies the non-unique cell identity. The controller further collectsmeasures and the associated non-unique cell identities and reports tothe radio base stations regularly. The controller is equipped with a CID(cell identity) detector 47 for identifying the non-unique cell identityin the physical layer information. The terminal blocks and functioningso far described are the same as for prior art terminals 4. However, thefunctioning of the controller is adapted for the present invention. Thecontroller has a request receiver 49, arranged for detecting a controlmessage from the serving base station on identifying the unique cellidentity of a cell of a non-unique cell identity. When such a requesthas been received, the controller 42, controls the transceiver 46 todetect data on the second cell broadcast channel, until the unique cellidentity has been identified. The controller 42 is equipped with a UCIDdetector that reads the information on the network layer, as receivedform the transceiver. The controller 42 controls the transceiver tocontinue detecting data on the broadcast channel until the UCID detectorhas found the unique cell identity. Owing to the unique cell identity isrepeated with long intervals, the identification of the unique cellidentity requires the transceiver to detect the broadcast channel longertime than if just then non-unique identity need be detected.

The CID detector 47, the UCID detector 48, and the request receiver 49are preferably implemented as SW modules, however, they mayalternatively be implemented in hardware or in a combination of the two.

Interactions with the user of the device take place using the manmachine interface 44, which can include a key pad, microphone,loudspeaker and display device, for example.

FIG. 4 b is a block diagram of a base station 2 which communicate via anair interface 6. Only blocks essential for the present invention aredisclosed. The base station 2 includes an input/output (I/O) interface24, a radio transceiver 26 an antenna 28 and a controller 22. Thecontroller communicates with mobile terminals via the transceiver 26 andantenna 28 over the air interface 6. The controller 22 also communicateswith the rest of the telecommunications network via the I/O interface24. For this invention the controller also includes NCL utility, 21, astorage device, 27, a temporary NCL, 23, and a NCL, 25. The controllerretrieves the NCL for identifying the unique cell identifiers for thenon-unique cell identifiers reported by the terminals. These aspects ofthe base station are designed to operate in accordance with usualpractice. The NCL unit monitors if the match between non-unique andunique cell identity can be made without ambiguity. If ambiguity existthe NCL unity initiates an instruction is sent to the terminal for it toreport of the unique cell identity of the non-uniquely identified cell.When the unique cell identity is reported by the terminal, the NCLutility 22 controls the unique and the non-unique cell identity arelisted and associated with the terminal identity in the temporary NCL. Afurther condition for listing in the temporary NCL is the reportedreception quality is above a first threshold level. The NCL utility 22also controls the non-relevant association to be deleted. Typically thisis made when the signal strength of a cell on the temporary NCL hasdecreased under a second threshold level. Alternatively the data base 27with NCL and temporary NCL are located outside the controller andconnected to it.

Typically the controller will also decide on handovers from the servingbase station to a target base station serving the second cell, or whichof the other cells that temporary will function as second cell.

In an alternative to the base station described with reference to FIG. 4b, the controller 22 is located in a radio network controller. The radionetwork controller with internal controller is then connected to atleast one radio base station transceiver. The functioning of theinternal controller 22 will then be the same as described in connectionto the radio base station.

However, base stations 2 and mobile terminals 4 which operate inaccordance with the present invention are adapted to carry out themethod of the invention as described below with reference to FIGS. 3 to6. It will be appreciated that the various functional units can beprovided by the controller 42, 22, or by other specific units in thedevices, or network 1.

Making use of unique cell identifiers (UCID) means that there isunambiguous information relating to the identity of the neighbour cells,and so confusion regarding those neighbouring cells is removed. Usingthe fast and low-resource demanding non-unique cell identity for most ofthe measurements, facilitates efficient resource usage within mobileterminals 4 and rapid handover to the neighbour cells. The mobileterminal 4 is only requested to retrieve the more cumbersome unique cellidentifier when a new neighbour is detected, or when an audit of therelation between the non-unique and unique cell identity seemsappropriate.

All cell relations can be continuously evaluated. Inputs to thatevaluation are mobile terminal reports and events, network events andoperator input. The result of the evaluation is that the cell or cellrelations will retain different properties. This can also be seen as thecell relation being in different states.

The major advantage of embodiments of the present invention is that itremoves the need for manual involvement within the process ofmaintaining neighbour sets. The operator can decide to fully neglect theconcept of neighbours and let the system take care of the neighbour celldefinitions.

1. A method for a radio base station or for a base station controllerconnected to a radio base station and controlling it wherein the radiobase station serves a first cell of a plurality of cells in a wirelesstelecommunications system, the method comprising the steps of:communicating with a mobile terminal in the first communications cell;receiving from the mobile terminal a quality measure on a second celland a non-unique identification of the second cell; retrieving aneighbour cell list for the first cell, to match a unique-cellidentification with the non-unique cell identity, and if ambiguity inthe match arises instructing the mobile terminal to determine the uniquecell identity of the second cell; receiving unique cell identifierinformation relating to the second communications cell from the mobileterminal; and associating the non-unique cell identity with the uniquecell identity.
 2. A method as claimed in claim 1, wherein ambiguityarises when the second cell non-unique identity is not included in theneighbour cell list.
 3. A method as claimed in claim 1, whereinambiguity arises if the second cell non-unique cell identity is includedin the neighbour cell list with information on suspected ambiguity ofidentification.
 4. A method as claimed in claim 1, wherein ambiguityarises if two unique cell identities in the list shares the samenon-unique cell identity.
 5. A method as claimed in claim 2, wherein insaid associating step said second cell non-unique and unique cellidentities is added to the neighbour cell list.
 6. A method as claimedin claim 3 or 4, wherein in said associating step said second cellnon-unique and unique cell identities is associated with thecommunication with the terminal for a period.
 7. A method as claimed inclaim 3 or 4, wherein in said associating step said second cellnon-unique and unique cell identities are associated with thecommunication with the terminal as long as the quality measure exceeds afirst threshold level
 8. A method as claimed in claim 3, whereininformation on suspected ambiguity is added to a non-unique cellidentity if one or more handovers from the first cell to a cell with thenon-unique cell identity has/have been unsuccessful.
 9. A method asclaimed in claim 1, wherein the quality measure is at least one of;signal strength measurement, a signal quality measurement, or timinginformation.
 10. A method as claimed in claim 1, wherein the instructingstep is performed on the further condition of the second cell qualitymeasure exceeds a second threshold value.
 11. A method as claimed inclaim 1, wherein the instructing step is performed on the furthercondition the second cell is determined to be a handover candidate. 12.A method as claimed in claim 1, comprising the further steps of:broadcasting a non-unique identity of the first cell, in a firstinformation layer intended for carrying information for supporting thephysical connection with the terminals; broadcasting a unique identityof the first cell on an information layer higher then the first layer.13. A radio base station for providing communication in a first cellwith at least one terminal and comprising: a radio transceiver; acontroller including or being connected to a database that includes aneighbour cell list and including a NCL utility, the controller beingarranged for receiving via the transceiver reports from the terminal onmeasured reception quality on other cells than the first cell, includingthe other cells non-unique cell identities, further arranged formatching the non-unique cell identities with corresponding unique cellidentities by retrieving the neighbour cell list, the NCL utilityarranged for initiating an instruction being sent to the terminal foridentifying and reporting on the unique cell identity of a non-uniquelyidentified cell if ambiguity arises in the matching, the NCL utilityfurther being arranged for associating the non-unique cell identity andthe unique cell identity as reported by the terminal
 14. A method for amobile terminal, 4, in a cellular telecommunications network, the methodcomprising: communicating, 301, over a first link in a first cell;measuring, 302, a signal quality of a channel broadcasted in a secondcell; detecting, 303, a non-unique cell identity broadcasted in thesecond cell: reporting, 304, the measured second cell signal quality andthe identified non-unique identity over the first radio link; receiving,305, an instruction over the first radio link to detect a second cellunique identity; detecting, 306 the unique cell identity broadcasted inthe second cell; reporting, 307, the unique second cell identity bytransmission over the first radio link.
 15. The method of claim 14,wherein the non-unique second cell identity is detected in a first layercarrying physical information.
 16. The method of claim 14, wherein theunique second cell identity is detected in a second layer higher thanthe first layer.
 17. A mobile terminal for communicating with a firstradio base station in a first cell and performing measures on thereception quality from other cells, and comprising: a radio transceiver,a controller arranged for identifying a non-unique cell identity of therespective other cells and report to the first radio base station withthe quality measures, and further arranged for controlling thetransceiver to identify a unique cell identity of one or more of theother cells in response to an instruction received from the first radiobase station.
 18. The mobile terminal of claim 17, wherein thecontroller includes a CID detector for identifying the non-unique cellidentity.
 19. The mobile terminal of claim 17, wherein the controllerincludes a UCID detector for identifying the unique cell identity.
 20. Aradio base station controller with an interface for connecting to atleast one radio base station and its transceiver and comprising: acontroller including or being connected to a database that includes aneighbour cell list and including a NCL utility, the controller beingarranged for receiving via the transceiver reports from at least oneterminal on measured reception quality on other cells than the firstcell, including the other cells non-unique cell identities, furtherarranged for matching the non-unique cell identity with a unique cellidentity by retrieving the neighbour cell list, the NCL utility arrangedfor initiating an instruction being sent to the terminal for identifyingand reporting on the unique cell identity of a non-uniquely identifiedcell if ambiguity arises in the matching, the NCL utility further beingarranged for associating the unique cell identity as reported by theterminal with the non-unique cell identity.